Research and developments on a wavelength division multiplexing (WDM) based passive optical network (PON) to provide voice, data, and broadcast convergent services that will be widely activated in the next few years are actively taking place throughout the world. The WDM based PON is referred to as the WDM-PON.
The WDM-PON is a method of communicating between a center office (CO) and subscribers using multiple wavelengths assigned to each subscriber. Since an exclusive wavelength is used for each subscriber, security is superior and large capacity communication service is possible. Also, a different transmission technology in terms of, for example, a link rate and a frame format, can be applied for each subscriber or each service through the single optical fiber.
However, since the WDM-PON network is a technology to multiplex various wavelengths in a single optical fiber using WDM technology, a number of different light sources as many as the number of subscribers belonging to a single remote node (RN) are needed. The production, installation, and management of a light source for each wavelength which act as a considerable economic burden to both users and or service providers are big obstacles to the commercialization of the WDM-PON. To solve the problem, a method of applying a wavelength tunable light source device that can selectively tune the wavelength of a light source is being widely studied.
As an example of the wavelength tunable light source, there is a wavelength tunable light source in the form of an external cavity laser (ECL) formed by arranging individual optical parts such as a semiconductor LD, a planar lightwave circuit (PLC), and an optical fiber. In a conventional ECL wavelength tunable light source, the individual optical parts are all mounted on a substrate and the optical coupling between the semiconductor LD and the PLC is made using a butt coupling method. Accordingly, various problems are generated which will be described in detail in a description portion with reference to FIGS. 8A and 8B.
The PLC device is used for the wavelength tunable light source. The PLC device has a structure in which light can propagate in the upper portion of a substrate such as silicon. In general, the structure for guiding light includes a core layer in which light propagates and a clad layer encompassing the core layer and having a refractivity about 0.0001-0.01 lower than that of the core layer. The PLC device which has a small device size and is compatible with a semiconductor process has superior productivity and is capable of performing various functions. For example, the PLC device is widely used for, for example, an optical power distributor, a wavelength splitting/combining filter, an optical switch using a thermo-optic effect, a variable optical attenuator, and a wavelength variable filter.
FIGS. 1A and 1B are a structural diagram and a functional block diagram of a conventional wave tunable light source (PLC-ECL) using a planar type thermo-optic device. Referring to FIG. 1A, the conventional PLC-ECL type wavelength tunable light source includes a reflective semiconductor optical amplifier (RSOA) 150 working as an optical gain medium, a PLC device 100 having a 3-D optical waveguide core layer 101, and an attachment optical fiber 160. The RSOA 150 has an resonator 151 similar to a semiconductor laser. In the RSOA 150, a front exit surface 152 and a rear exit surface 153 are respectively coated with a non-reflective film and a high reflective film. Thus, since a self laser oscillation is restricted, the RSOA 150 functions as an optical gain medium.
Although not only the RSOA but also a reflective laser diode (R-LD) can be used as the optical gain medium, in the present description, the RSOA is mainly referred to a the optical gain medium for the convenience of explanation. A Bragg grating 102 is formed in a part of the 3-D optical waveguide core layer 101 of the PLC device 100. A thin film metal heater 103 is arranged close to the Bragg grating 102. When the RSOA 150 is driven after the 3-D optical waveguide core layer 101 of the PLC device 100 is optically coupled to the resonator 151 of the RSOA 150, the ECL is formed between the Bragg grating 102 and the high reflective film of the rear exit surface 153 and a laser having a wavelength matching an effective period of the Bragg grating 102 is oscillated. When the light output of the PLC device 100 is coupled to the attachment optical fiber 160, a light source applicable to an external optical communication network is produced.
When current is applied to electrodes 105 at both ends of the thin film metal heater 103, the heat generated from the thin film metal heater 103 increases the temperature of the 3-D optical waveguide core layer 101 that is adjacent thereto. Accordingly, the refractivity of the 3-D optical waveguide core layer 101 is decreased by the thermo-optic effect, which shortens the effective period of the Bragg grating 102 so that the output light wavelength of a PLC-ECL 170 of FIG. 1B is varied toward the short wavelength. The Bragg grating 102 and the electrodes 105 at both ends of the thin film metal heater 103 constitute a wavelength tunable area 107. A thin film metal heater 104 arranged at the 3-D optical waveguide core layer 101 where the Bragg grating 102 is not formed and electrodes 106 at both ends of the thin film metal heater 104 constitute a phase control area 108. The phase control area 108 controls a round trip phase of the output light wavelength of the PLC-ECL 170 selected by the wavelength tunable area 107.
Referring to FIG. 1B, the PLC-ECL 170 includes the RSOA 150 as an optical gain medium, the phase control area 108, the wavelength tunable area 107, and the attachment optical fiber 160 according to the function thereof. In the following description, the phase control area 108 is omitted for the convenience of explanation.
FIG. 2A is a perspective view of a conventional 3-D optical waveguide type thermo-optic device, that is, the wavelength tunable area of the wavelength tunable light source of FIG. 1A. Referring to FIG. 2A, the wavelength tunable area 107 includes an underclad layer 111 provided on a silicon substrate 110, the 3-D optical waveguide core layer 101 where a core layer is formed in a 3-D rod shape, and an overclad layer 112 covering the upper portion of the 3-D optical waveguide core layer 101. In FIG. 2A, the thin film metal heater arranged close to the light waveguide is not illustrated.
The thickness tunable Bragg grating 102 formed in an interference exposure-etching method is provided in part of the upper portion of the 3-D optical waveguide core layer 101. The Bragg grating 102 reflects a wavelength component corresponding to twice the effective Bragg grating period with respect to the light propagated in the 3-D optical waveguide core layer 101, thus forming an ECL oscillator with respect to a corresponding wavelength. The 3-D optical waveguide core layer 101 and the clad layers 111 and 112 can be manufactured of various materials such as a semiconductor material, a dielectric material, and a polymer material.
FIG. 2B is a graph showing the thermo-optic effect of the optical waveguide type thermo-optic device of FIG. 2A. In FIG. 2A, the thermo-optic effect when the optical waveguide type thermo-optic device of FIG. 2A is manufactured of a polymer material is shown. Referring to FIG. 2A, a polymer used as an optical waveguide generally has a thermo-optic coefficient or a coefficient of thermal expansion (CTE) of about (−0.7˜−2.2)×10−4/° C. The thermo-optic coefficient of the polymer used in the experiment is about −1.822×10−4/° C. That is, the refractivity of the optical waveguide formed of the polymer decreases as temperature increases. Accordingly, the effective period of the Bragg grating 102 is reduced so that the output optical wavelength of the PLC-ECL 170 is varied toward a short wavelength.
FIGS. 3A and 3B are a cross-sectional view and a front view of a thin film metal is heater portion of the conventional PLC. Referring to FIG. 3A, the conventional PLC de vice includes a silicon substrate 110, an underclad layer 111 provided on the silicon substrate 110, the 3-D optical waveguide core layer 101 where the Bragg grating 102 is formed, the overclad layer 112 provided on the 3-D optical waveguide core layer 101, and the thin film metal heater 103 arranged on the surface of the overclad layer 112.
In the PLC structure, when current is applied to the thin film metal heater 103, the temperature of the 3-D optical waveguide core layer 101 existing under the thin film metal heater 103 is partially increased. The refractivity of the 3-D optical waveguide core layer 101 is changed in proportion to the amount of change in temperature (DT) according to the thermo-optic coefficient of the optical waveguide material. Typically, in the temperature change amount DT, a reflectivity change amount Dn according to the thermo-optic coefficient is expressed by the following equation.Dn=CTE×DT  [Equation 1]
For the substrate 110, the temperature is maintained at a constant level by using a thermo-electric cooler (TEC) device or attaching a heat dissipating plate to prevent the temperature of the substrate 110 from being changed over time.
The thin film metal heater 103 is typically manufactured of chrome, nickel, nichrome, tungsten, and tungsten silicide and formed on the surface of the overclad layer 112 of the optical waveguide. The temperature of the 3-D optical waveguide core layer 101 is increased by applying current to the thin film metal heater 103 in a state in which the temperature of the substrate 110 is maintained at a constant level. The PLC configured as above has the following problem.
FIG. 4 is a graph showing the distribution of the temperature in the vertical direction of the thin film metal heater of the PLC device of FIG. 3A. Referring to FIG. 4, the temperature of the thin film metal heater 103 increases as the current applied to the thin film metal heater 103 increases. However, since the temperature of the substrate 110 is maintained at a constant level, the transfer of the temperature to the 3-D optical waveguide core layer 101 linearly decreases. That is, in the conventional PLC structure, since the temperature of the optical waveguide cannot be changed much through the heater, it is a disadvantage that the width of a tunable wavelength is narrow. In particular, the inclination of temperature (hereinafter, referred to as the temperature gradient) increases as the current applied to the thin film metal heater 103 increases. A high temperature gradient deforms the distribution of the refractivity around the 3-D optical waveguide core layer 101. Accordingly, when light propagates, light dispersion, a higher mode generating, and optical loss are generated so that the characteristic of the optical waveguide is degraded.
FIGS. 8A and 8B respectively are a plan view and a side view of a conventional wavelength tunable light source. Referring to FIGS. 8A and 8B, the wavelength tunable light source includes the PLC device 100, the RSOA 150, and the attachment optical fiber 160. In the conventional wavelength tunable light source, a silicon optical bench 201 having an RSOA assembly area 204 and an optical fiber assembly area 203 where a V-groove is formed, which are at both ends of the PLC device 100, is packaged in a butterfly type package 200. The attachment optical fiber 160 is assembled on the optical fiber assembly area 203 to be aligned to the 3-D optical waveguide core layer 101 of the PLC device 100 using the V-groove. The RSOA 150 is assembled on a pad arranged to be aligned to the 3-D optical waveguide core layer 101 of the PLC device 100, that is, the RSOA assembly area 204, in a flip chip bonding method. A thermo-electric cooler (TEC) 202 is attached to a lower portion of the silicon substrate, that is, the silicon optical bench 201, to maintain a constant temperature. A thermistor 205 is attached to an upper portion of the silicon substrate to monitor the temperature of the surface of the silicon substrate. An electrode pad in the package is electrically connected to a package lead 207 through a bonding wire 208.
The conventional wavelength tunable light source has the following problems in view of an optical coupling efficiency, a performance efficiency, and mass productivity that are major considerations in packaging of a PLC-ECL wavelength tunable light source. First, since the optical coupling among the RSOA 150, the 3-D optical waveguide core layer 101, the attachment optical fiber 160 is butt-coupling not using a lens, an optical coupling efficiency can be reduced to ½ at its maximum compared to a case of using a lens. In addition, the optical coupling efficiency is further degraded considering the flip chip bonding and the alignment error (1-2 μm) of the v-groove.
Second, since various parts constituting the wavelength tunable light source are mounted on a single substrate, it is not possible to selectively combine each of the functional portions with their best parts. Furthermore, when the performance of a part of the finally assembled light source, for example, the optical coupling efficiency between the RSOA and the PLC device, is low, the performance of the overall light source is degraded so that it is difficult to guarantee a performance quality and production yield.
Third, since various parts constituting the wavelength tunable light source are mounted on a single substrate, not only a total production yield is low but also repair and restoration is impossible when a problem is generated during the process. This causes a considerable burden in the quality management for device production. Thus, yield of an optical module device is lowered so that lowering of a price is very difficult.
The wavelength assigned to each subscriber node in the WDM-PON is determined by a wavelength passing through an arrayed wavelength grating (AWG) connected to the subscriber node. Accordingly, the WDM-PON system needs to support a series of initialization functions to align wavelengths to assigned intrinsic wavelengths when the network is connected to a subscriber node. Of the initialization methods, a method of determining the wavelength of an ONT (optical network terminal) based on an optical signal transmitted from an OLT (optical line terminal) to the ONT is most preferred.
In this case, an optical transmitter used for the ONT cannot use an independent light source that can self-oscillate but uses a separate seed light source provided by the OLT so as to use mode locking type light source or a reflection type light source. For such a source, an additional wavelength initialization function is not needed because an input light wavelength is used as it is. However, this method is applied only to the RSOA or an injection locking based FP-LD using a locking or reflection mechanism. Furthermore, the initialization function cannot be added to the wavelength tunable light source having a self-oscillation function.